Fatty acids, or the products derived from them, are valuable as food additives, dietary supplements, specialty chemicals, lubricants, fuels, and petroleum substitutes. Fatty acids can be generally classified as straight-chain fatty acids or non-straight-chain fatty acids. Whereas straight-chain fatty acids are relatively abundant, non-straight-chain fatty acids are not. Important classes of non-straight-chain fatty acids include branched-chain fatty acids, furan-containing fatty acids, and cyclic fatty acids.
Branched-chain fatty acids are constituents of the lipids of bacteria and animals. They are sometimes found in the integral lipids of higher plants. The fatty acyl chain on branched-chain fatty acids may be saturated or unsaturated. The branch may be methyl or a higher-order branch. The most common branched-chain fatty acids are mono-branched, but di- and poly-branched fatty acids also occur and may be either saturated or unsaturated.
Branched-chain fatty acids are known to have additional preferred properties when compared to straight-chain fatty acids of the same molecular weight (i.e., isomers), such as considerably lower melting points which can in turn confer lower pour points when made into industrial chemicals. These additional benefits allow the branched-chain fatty acids to confer substantially lower volatility and vapor pressure and improved stability against oxidation and rancidity. These properties make branched-chain fatty acids particularly suited as components for feedstock for cosmetic and pharmaceutical applications, or as components of plasticizers for synthetic resins, solvents for solutions for printing ink and specialty inks, and industrial lubricants or fuel additives.
Furan-containing fatty acids are a large group of fatty acids characterized by a furan ring. The furan ring typically carries at one α-position an unbranched fatty acid chain with 9, 11, or 13 carbon atoms and at the other α-position a short straight-chain alkyl group with 3 or 5 carbon atoms (Glass et al. 1975). In most cases, both β-positions of the furan ring are substituted by either one or two methyl residues or other groups. Furan-containing fatty acids without any substitutions on the β-positions of the furan ring also occur (Morris et al. 1966). Furan-containing fatty acids are widely distributed in nature as trace components of plants, fishes, amphibians, reptiles, microorganisms, and mammals, including humans (Glass et al. 1975, Glass et al. 1974, Gunstone et al. 1978, Hannemann et al. 1989, Ishii et al. 1988, Ota et al. 1992).
Furan-containing fatty acids appear to be involved in various important biological functions and act in an antioxidant, antitumoral, and antithrombotic capacity (Ishii et al. 1989, Graft et al. 1984, Okada et al. 1996). The correlation between consumption of fish rich in furan-containing fatty acids and protection against coronary heart disease mortality has been confirmed in several studies (Spiteller 2005). Furan-containing fatty acids have also been reported to have inhibitory effects on blood platelet aggregation (Graft et al. 1984) and to have potential antitumor activity (Ishii et al. 1988). Furan-containing fatty acids prevent oxidation of linoleic acid (Okada et al. 1990) and act as antioxidants in plants (Batna et al. 1994). Some studies have demonstrated that furan-containing fatty acids undergo oxidation by ring opening to form dioxoenes (Jandke et al. 1988, Schodel et al. 1985) in the presence of linoleic acid as a co-substrate, indicating that that furan-containing fatty acids act as radical scavengers (Fuchs et al. 2000, Halliwell et al. 1990). These effects of furan-containing fatty acids make them valuable as dietary supplements for animals, including humans.
Furan-containing fatty acids also have potential use as advanced biofuels, oxygenates, or fuel additives. The presence of the oxygen atom in the fatty acyl chain provides a reactive group for catalytic conversion to branched acyl chains that are useful as fuels. The presence of the oxygen in a hydrocarbon backbone may also enhance combustion or provide a site to control radicals that are formed during fuel combustion (Rothamer et al. 2013).
Cyclic fatty acids typically comprise a 3- to 7-membered ring in the hydrocarbon chain or at the terminus of the hydrocarbon chain. The ring may be saturated (cyclopropane, for example) or unsaturated (cyclopropene, for example). Cyclic fatty acids occur naturally in plants, especially certain seed oils, and microorganisms, but only rarely in animal tissues. Cyclic fatty acids include cyclopropane fatty acids, such as lactobacillic acid and majusculoic acid; cyclopropene fatty acids such as sterculic acid and malvalic acid; and fatty acids with terminal ring structures, such as 11-cyclohexylundecanoic acid, 13-cyclohexyltridecanoic acid, 2-hydroxy-11-cyclohepylundecanoic acid, ladderane fatty acids, chaulmoogric acid, and gorlic acid.
Strategies for obtaining non-straight-chain fatty acids at high quantities are needed.